The invention relates to chemical fuel cells. More particularly, the invention relates to membranes used in the membrane electrode assemblies of chemical fuel cells.
Chemical fuel cells utilize renewable resources and provide an alternative to burning fossil fuels to generate power. Fuel cells utilize the oxidation/reduction potentials of chemical reactions to produce electrical current.
For example, methanol is a known example of a fuel source used in chemical fuel cells. In a methanol driven fuel cell, methanol and water can be circulated past an anode that is separated from a cathode by a membrane that is selectively permeable to protons. The following chemical reaction takes place at the anode.
Anode:CH3OH+H2Oxe2x86x92CO2+6H++6exe2x88x92
The protons generated at the anode pass through the membrane to the cathode side of the fuel cell. The electrons generated at the anode travel to the cathode side of the fuel cell by passing through an external load that connects the anode and cathode. Air or an alternative oxygen source is present at the cathode where the electro-reduction of oxygen occurs resulting in the following chemical reaction.
Cathode:1.502+6H++6exe2x88x92xe2x86x923H2O
One of the key aspects of a chemical fuel cell is the membrane-electrode assembly (MEA). The MEA typically includes a selectively permeable polymer electrolyte membrane bonded between two electrodes, e.g., an anode electrode and a cathode electrode. The materials chosen for constructing the membrane should allow protons to pass through the membrane and prevent the fuel sources from passing through the membrane.
When the fuel, e.g., methanol, permeates the membrane and combines with oxygen on the cathode side of the fuel cell, the overall operating potential of the fuel cells is diminished. This phenomenon is termed fuel crossover. The rate of crossover is a parasitic reaction that is proportional to the permeability of the fuel through the membrane and increases with increasing fuel concentration and temperature. Thus, choosing the appropriate membrane material can increase the overall fuel cell performance.
One currently preferred resin of choice for fabricating MEAs has been NAFION ((trademark)), which is a co-polymer of a tetrafluoroethylene and perfluorpolyether sulfonic acid made by DuPont De Nemours. NAFION ((trademark)) membranes are chemically stable, strong and reasonably conductive. There are at least two drawbacks associated with NAFION ((trademark)). NAFION ((trademark)) is extremely expensive. Further, NAFION ((trademark)) suffers from fuel crossover. Accordingly, there exists a need for improved and cost-effective fuel cell electrolyte membranes.
In one aspect, the present invention provides an electrolyte membrane for use in a fuel cell that contains sulfonated polyphenylether sulfones.
In one embodiment, the membrane contains a first sulfonated polyphenylether sulfone and a second sulfonated polyphenylether sulfone, wherein the first sulfonated polyphenylether and the second sulfonated polyphenylether sulfone have equivalent weights greater than about 560, and the first sulfonated polyphenylether and the second sulfonated polyphenylether sulfone also have different equivalent weights.
In another embodiment, the sulfonated polyphenylether sulfones of the membrane can have equivalent weights that differ by at least about 50 equivalent weight units. The first and second sulfonated polyphenylether sulfones can have equivalent weights ranging from about 560 to about 720. Further, one of the sulfonated polyphenylether sulfones can have an equivalent weight greater than about 1000. Accordingly, there can be a membrane wherein the first sulfonated polyphenylether sulfone has an equivalent weight greater than about 1000, and the second sulfonated polyphenylether sulfone has an equivalent weight ranging from about 560 to about 720.
In another embodiment, the relative amounts of the sulfonated polyphenylether sulfones can be altered so that the first sulfonated polyphenylether sulfone is at least about 70% of the membrane on a weight by weight basis. The first sulfonated polyphenylether sulfone can be at least about 70% of the membrane on a weight by weight basis when the second sulfonated polyphenylether sulfone is no greater than about 30% of the membrane on a weight by weight basis. As such, it is possible to have a membrane where the first sulfonated polyphenylether sulfone is about 70% of the membrane on a weight by weight basis, and the second sulfonated polyphenylether sulfone is about 30% of the membrane on a weight by weight basis.
In another embodiment, a membrane for use in a fuel cell can contain a sulfonated polyphenylether sulfone and an unsulfonated polyphenylether sulfone. The sulfonated polyphenylether sulfone can have an equivalent weight of at least about 560, ranging from about 560 to about 720. The sulfonated polyphenylether sulfone can encompass at least about 75% of the total weight of the membrane on a weight by weight basis. When the sulfonated polyphenylether sulfone is at least about 75% of the total weight of the membrane on a weight by weight basis, the unsulfonated polyphenylether sulfone can be no greater than about 25% of the total weight of the membrane on a weight by weight basis.
The unsulfonated polyphenylether sulfone can be the base polymer of the sulfonated polyphenylether sulfone. In such an instance, it is possible for the sulfonated polyphenylether sulfone to have an equivalent weight of at least about 560 or an equivalent weight ranging from about 560 to about 720.
The sulfonated polyphenylether sulfone can be at least about 75% of the total weight of the membrane on a weight by weight basis. When the sulfonated polyphenylether sulfone is at least about 75% of the total weight of the membrane on a weight by weight basis, it is possible for the unsulfonated polyphenylether sulfone to be no greater than about 25% of the total weight of the membrane on a weight by weight basis.
In a second aspect, the invention features a fuel cell that includes a fuel supply, an anode assembly, a cathode assembly, and a membrane containing a sulfonated polyphenylether sulfone and an unsulfonated polyphenylether sulfone, wherein the membrane can be bonded between the anode assembly and the cathode assembly.
In a third aspect, the invention features a fuel cell that includes a fuel supply, an anode assembly, a cathode assembly, and a membrane containing a first sulfonated polyphenylether sulfone and a second sulfonated polyphenylether sulfone, wherein the first sulfonated polyphenylether and the second sulfonated polyphenylether sulfone can have equivalent weights greater than about 560, and the first sulfonated polyphenylether and the second sulfonated polyphenylether sulfone can have different equivalent weights, and wherein the membrane can be bonded between the anode assembly and the cathode assembly.
In a fourth aspect, the invention features a method for manufacturing a membrane for use in a fuel cell that includes the steps of mixing at least one sulfonated polyphenylether sulfone polymer with an unsulfonated polyphenylether sulfone polymer to form a blend, casting the blend for a first time, dissolving the blend after the first casting in a solvent, and casting the blend for a second time, the blend forming a visually stable membrane. The solvent can include dimethylformamide.
In a fifth aspect, the invention features a method for manufacturing a membrane electrode assembly for use in a fuel cell that includes the steps of contacting a membrane containing a blend of a sulfonated polyphenylether sulfone polymer and an unsulfonated polyphenylether sulfone polymer with a catalyst thereby forming a membrane bonding side, coating an electrode support with an electrolyte polymer and a catalyst thereby forming an electrode bonding side, contacting the membrane bonding side and the electrode bonding side, and bonding the membrane bonding side and the electrode bonding side. The electrolyte polymer can include a liquid mixture of a co-polymer of a tetrafluoroethylene and a polyhydrocarbon sulfonic acid and a catalyst. The polyhydrocarbon sulfonic acid can be fluorinated and can include, for example, perfluoropolyether sulfonic acid.
The method may include roughening a membrane surface before contacting said membrane with said catalyst. The roughening can be performed by sanding the membrane.
In a sixth aspect, the invention features a method for fabricating an electrode for use in a chemical fuel cell that includes the steps of contacting an electrode support with a catalyst, and sintering the electrode that now contains a catalyst. The sintering of the electrode can be performed in a nitrogen environment.
In a seventh aspect, the invention features a method for fabricating an electrode for use in a chemical fuel cell that includes the steps of contacting an electrode support with a first catalyst, sintering the electrode support containing the first catalyst, and contacting the sintered electrode with a second catalyst. The second catalyst can further include an electrolyte polymer.
In an eighth aspect, the invention features an electrode for use in a fuel cell that includes a catalyst contacting a side of an electrode support, wherein the electrode can be sintered after contacting the catalyst. The electrode can further include another catalyst contacting the electrode after the electrode is sintered.
In a ninth aspect, the invention features a fuel cell that includes a fuel supply, a membrane, and an electrode that can include a sintered electrode support containing a catalyst on a side of the sintered electrode support, wherein the electrode can be sintered after contacting the catalyst. In another embodiment, the fuel cell can include further a catalyst contacting the sintered electrode.
The invention provides membranes and fuel cells that can be constructed from relatively inexpensive polyphenylether sulfones. Further, the membranes and electrodes can provide improved fuel cell performance by diminishing parasitic crossover reactions and sustaining voltage differentials across membrane electrode assemblies.